youtube: fix, clean and document the YouTube benchmark
youtube: fix, clean and document the YouTube benchmark

 - use _target private member (according to new android benchmark API)
 - set brightness to 0 during benchmark run
 - sync imports for consistency with other workloads (Jankbench)
 - add documentation for the YouTube run method.

Signed-off-by: Ionela Voinescu <721bf4aa4c535b5f5f0a38815c215c0ece88733b@arm.com>

joelagnel/lisa,credp/lisa,mdigiorgio/lisa,credp/lisa,ARM-software/lisa,bjackman/lisa,arnoldlu/lisa,ARM-software/lisa,arnoldlu/lisa,joelagnel/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,mdigiorgio/lisa,bjackman/lisa